Unable to merge two objects
From the pictures you've provided, it appears that you do not have the same number of vertical spans for the head mesh and the neck mesh. They will need to be the same before they can be properly joined. You won't want to count them manually, so just select each of the edge loops in Edit Mode individually, and then look to the bottom right of the screen to see the number of edges and vertices for each of the selected edge loops. This will tell you exactly how many more spans one mesh has than the other. It looks to me as if your neck mesh is the one which has more.

If you don't see this information, right-click on the field where it should appear, and a menu will appear. Check the box that says Scene Statistics. Now you can see the information.
Fixing your kind of topology problem can be a headache. You could try to remesh the parts to fix it, but I find that it's more straightforward to simply manually select and delete the extra spans. If you are worried about how that may damage your model's shape, you can try to retain it by shrinkwrapping your body mesh to an unedited copy of the body mesh, then delete the spans for your project body mesh, and then adjust the remaining spans on it with by edge-sliding the spans into somewhat better positions. Shrinkwrapping to the copy mesh ensures that the overall shape will be preserved.
If you don't want to delete any spans, there is another way to get this done. You could add spans to the head instead of removing them from the body. One nice thing about going this route is that it won't interfere with your shape. But it will create unnecessary geometry, and maybe even ugly topology. You could consider using the Knife tool to add small edges that bring the total count up on the head side of the join, but then those new edges you create will have to end up going somewhere. There is a trick that prevents you from needing to follow a newly created span all the way around the object you're adding it to. You can get rid of any two extra spans at the same time by tracing them into the new mesh area and then making sure they meet one another at the centerline of the mesh. To keep things all quads, though, you will want the vertex that defines the 90 degree bend that this solution requires to be joined to the nearest corner vertex. It looks ugly, but it gets the job done (Newly added knife cuts shown in dotted green; centerline shown in dotted red).

Your neck mesh looks especially problematic from the side. You may have to use loop tools (an addon which is included with Blender, but not on by default) to straighten and round out the neck hole. You should probably also partially rebuild the topology on the side of the neck so that the edge flow is consistent with that of the head.

When you finally get the spans to be the same number for both parts, your bridging of the edge loops should work. But be aware that you must be sure to join your mesh parts before beginning this operation.